Transmission techniques requiring knowledge about the channel used for transmission comprise two stages, as shown in FIG. 1 for an SDMA technique: a stage F of estimating the channel by recovering information about the channel by feedback, without transmitting payload data, and a stage T of transmitting payload data.
During the feedback stage, the transmitter, e.g. an access point AP, sends a packet having pilots, referred to as a “sounding” packet, to stations (STA#1, STA#2) that can estimate the downlink channel DL. These stations then encode information about the estimated channel, written CSI_DL, and transmit that information to the access point AP in a frame written “CSI report”. That mode of channel estimation is sometimes referred to as “explicit feedback”.
During the transmission stage, the access point AP sends one or more payload data packets “DL-MU-MIMO” to a plurality of users, e.g. by making use of a particular beam towards each user. FIG. 1 shows a base element, i.e. a payload data transmission packet, followed by acknowledgments of the reception of the payload data, which acknowledgments are transmitted by the users. The base element may be duplicated as often as desired, so long as the channel does not vary. This variation over time in the channel thus determines the duration of a transmission stage. Thus, if the channel varies sufficiently over 25 milliseconds (ms) for the SDMA technique to cease functioning, e.g. because interference between users has become too strong, then the duration of a transmission stage must be less than 25 ms.
At the end of a transmission stage, a new feedback stage is performed in order to obtain an up-to-date estimate of the transmission channel.
It should thus be observed that the longer the payload data transmission stage, the greater the overall transmission data rate.
Nevertheless, as mentioned above, the length of the second stage is constrained by variations in the channel over time, due to movements of the transmitter or of the receiver, or merely to certain movements in the environment.
For example, the beam-forming technique of the standard 802.11n that serves to focus the transmission power towards a particular receiver and thus increase the signal-to-noise ratio on reception, performs a feedback stage once every 100 ms in open space and when there is no movement of the transmitters and/or receivers.
In the same environment, when using the multi-user SDMA technique, this period for performing the feedback stage is shortened to 25 ms.
Since the period for performing the feedback stage has great influence on the overall transmission data rate, it is important to lengthen it as much as possible, i.e. to increase the duration of the transmission stage between two feedback stages.
Several techniques exist for modifying the ratio between the durations of the feedback and transmission stages for the purpose of optimizing the overall transmission data rate.
A first category of techniques consists in reducing the duration of the feedback stage by compressing the channel information. Nevertheless, such compression is often detrimental to transmission quality.
A second category of techniques consists in lengthening the duration of the transmission stage.
For example, with the SDMA technique, an access point AP having four antennas can transmit simultaneously on the same frequency band to four users/receivers, by forming a beam towards each user so as to eliminate interference between users. The beams are calculated on the basis of the channel information for each user.
Under ideal circumstances, each user receives only the information addressed to that user, i.e. one space-divided stream from among the four space-divided streams sent by the access point.
In a first mode, the access point thus sends to each user only the pilot symbols needed for estimating the channel of the space-division stream addressed to that user.
Nevertheless, for various reasons, including for example variation of the channel over time, a user may receive information addressed to that user together with interference from information addressed to other users.
If a user possesses more receive antennas than the number of space-divided streams that are addressed to that user, it is possible for the user to eliminate some or all of the information by interference canceling techniques (such as the minimum mean square error (MMSE) technique). Nevertheless, this is possible only if the user is capable of estimating the channels of the space-divided streams addressed to the other users.
In a second mode, in order to able to eliminate all or some of the interference, the access point thus transmits to each user both the symbols enabling the user to estimate the channel for the space-divided stream addressed to that user, and also pilot symbols enabling each user to estimate the channels for the space-divided streams addressed to the other users. This second mode makes it possible to be more robust in the face of channel variations over time, since it enables the interference generated thereby to be canceled. Naturally, as soon as the interference becomes excessive, it is necessary to perform a new feedback stage.
Nevertheless, a drawback of interference canceling techniques lies in the need to transmit more pilot symbols to each user, thereby reducing the efficiency of the physical layer of the transmission system.
Another drawback of those techniques is due to the fact that, at user level, it is necessary to have at least one more antenna than there are space-divided streams to receive. This gives rise to greater complexity and greater cost at receiver level. Insofar as the receivers are more and more frequently mobile stations, this drawback can become very considerable.
There therefore exists a need for a novel transmission and/or reception technique for use in transmission systems that require knowledge about the channel at the time of transmission in order to optimize the duration of the transmission stage between two feedback stages.